meson: simplify dependencies

The preferred way for identifying dependency fallback variables is
for the subproject wrap files to express them.  Move from the
meson.build to the corresponding wrap files.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I83284ee1c6892503ff436c8280c1c5aeaf4149c0
diff --git a/subprojects/sdeventplus.wrap b/subprojects/sdeventplus.wrap
index 085bb5e..f871ac0 100644
--- a/subprojects/sdeventplus.wrap
+++ b/subprojects/sdeventplus.wrap
@@ -1,3 +1,6 @@
 [wrap-git]
 url = https://github.com/openbmc/sdeventplus.git
 revision = HEAD
+
+[provide]
+sdeventplus = sdeventplus_dep